This invention relates to a method and an apparatus for liquid crystal display capable of displaying intermediate tones partial tones or half tones of images.
A known method for displaying an intermediate tone that is less than a normal or full tone of an image on a liquid crystal display panel employs a fast blinking operation as disclosed, for example, in JP-A-58-57192. This conventional technique will first be described using FIGS. 7 through 10.
FIG. 7 shows in block diagram the conventional liquid crystal display apparatus, which includes a display address generating circuit 1, memories 2 and 3, a switching signal generating circuit 4, a memory switching circuit 5, a display data forming circuit 6, and a liquid crystal display panel 7.
In the arrangement, a display address 8 issued by the display address generating circuit 1 is received by the memory 2 and memory 3 simultaneously, and the memories 2 and 3 read out character codes. The switching signal generating circuit 4 provides a switching signal to the memory switching circuit 5, and when the signal is "high", a character code read out of the memory 2 is fed to the display data forming circuit 6 by way of the memory switching circuit 5, while when the switching signal is "low", a character code read out of the memory 3 is fed through the memory switching circuit 5 to the display data forming circuit 6. The switching signal alternates its binary levels in every display period for consecutive frames on the liquid crystal display panel 7, so that when the memory 2 is selected by the memory switching circuit 5 to supply its contents to the display data forming circuit 6 at the time of displaying the first frame, the memory 3 is selected next at the time of displaying the second frame. The display data forming circuit 6 forms a supplied character code into a character pattern and delivers it as a display data 9 to the liquid crystal panel 7.
Supposing characters "A", "B" and "C" are displayed on the liquid crystal panel 7 with the character "B" being displayed in an intermediate tone, the memory 2 stores codes A, B and C representing characters "A", "B" and "C", whereas the memory 3 stores only character codes A and C. On this account, when the memory switching circuit 5 selects the memories 2 and 3 alternately, the characters "A", "B" and "C" are displayed in the first frame as shown in FIG. 9A, while only characters "A" and "C" are displayed in the second frame as shown in FIG. 9B. Accordingly, the characters "A" and "C" are displayed in every frame, while the character "B" is displayed in every two frame, resulting in an intermediate tone for the character "B" as shown in FIG. 9C. However, when the display apparatus operates at a typical frame frequency of 60 Hz, the character "B" appears iteratively at 30 Hz, which causes a pronounced flicker as a result of the intermediate tone display.
Application of a d.c. voltage to liquid crystal brings on electrolysis, which impairs the operating life of the device. Therefore, an alternating display data signal must be supplied to the liquid crystal panel so as to avoid a defect caused by d.c. voltage application. For this reason, the liquid crystal panel 7 is given an alternating signal so that the display data signal has alternate polarities for consecutive frames, although this aspect is not shown in FIG. 7. Namely, the first, third and fifth frames have display data signals with a positive polarity, while the second, fourth and sixth frames have display signals with a negative polarity, as shown in FIG. 10.
In the prior art liquid crystal display apparatus producing an intermediate tone, as illustrated in FIG. 7, no display data signal is supplied in even-numbered frames to the display area where the character "B" is to be displayed in an intermediate tone, and the display data signals for these frames have polarities of "+", ".+-.0", "+", ".+-.0", "+", and so on as shown in FIG. 10. This portion of the liquid crystal panel is applied with the voltage signal only in odd-numbered frames with a positive polarity invariably, and this means the application of a d.c. voltage to liquid crystal due to the integration effect, resulting in an impaired service life of the liquid crystal display panel.